Frequency modulation reception



Nov. 22, 1949 H. TUNlcK FREQUENCY MODULATION RECEPTION 4 Sheets-Sheet l Filed March 9, 1940 Nov. 22, 1949 H. TUNICK FREQUENCY MODULATION RECEPTION Filed March 9, 1940 4 Sheets-Sheet 2 www Nov. 22, 1949 H. TUNlcK FREQUENCY MODULATION RECEPTION Filed March 9, 1940 4 Sheets-Sheet 3 INVENTOR. HARRY TUN/CK a gina ORIVEY.

Nov. 22, 1949 H. TUNlcK 2,488,612

FREQUENCY MODULATION RECEPTION Filed March 9, 1940 4 Sheets-Sheet 4 Patented Nov. 22, 1949 FREQUENCY MODULATION RECEPTION Harry Tuniek, Rye, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application March 9, 1940, Serial N0. 323,089

2 Claims. (Cl. Z50-20) My present invention relates to receivers for receiving and translating frequency modulated waves. The principal object of my present invention is to provide an improved frequency modulation receiver in which the pass Yband is automatically varied in accordance with the frequency swing of the received waves. In this manner, as will be described more fully later, I provide a receiver having an improved signal to noise ratio.Y

Figures 8 and 9 are modifications of the re-V ceiver of Figure 5,

Fig. 10 shows another modification of the invention. Figures 1 to 7 inclusive are, respectively, Figures 3, 3a, 3b, 3c, 4, 4a and 4b of my application Serial No. 310,495, led December 22, 1939, now

Patent No. 2,296,962, granted September 29, 1942. f

In Hansell Patent No. 1,819,508 there is described a frequency modulation signaling system in which the frequency deviation at the transmitter or the frequency channel employed is made greater or smaller and the receiver is similarly adjusted so as to accommodate the transmitter swing. As shown in Figure 2 when the Hansell system is adjusted for the narrower swing in frequency, a swing in frequency fl will produce a signaling currentin the detector Is. When thel Hansell system is adjusted to a wider swing a deviation at the transmitter of f2, as shown in Figure 3, will be required to produce the same current Is in the radio receiver. The background noise in both cases is the sameand may be `represented by a change in frequency of the carrier equal to df. That is, as the carrier is swung in frequency with the modulating signal, the background noise gives the carrier a, supplemental vibration or quiver equal to df which isv productive of the noise in the receiver. In Figure 2 when the system is adjusted for narrow swings the noise current produced in the receiver is equal to dIa. In this case then the signal to noise ratio is given by the ratio of Is to dla. y

As shown in the Hansel] patent, when the wider swings are employed, and as illustrated in Figure 3, the wiggle in carrier frequency df produced by the noise produces a noise current in the receiver equal to dIb forthe same receiver or detector output Is. The signal to noise ratio in this case, shown by Figure 3, is, therefore, Is divided by dIb.

Since the noise current dIa in the case of Figure 2 operation is equal to elf-tan A and since in the case of Figure 3 dIb is equal to df tan B, it follows that dIa is to dIb as f2 is to fl.

In other words, the constant background noise is reduced in direct proportion to the swing in frequency employed in the system.

In either system it should also be clear that the signal to noise ratio improves with louder signals; that is to say, in either system the current produced in the receiver is directly proportional to the frequency f and since the noise current represented by background noise is substantially constant, there is a better signal to noise ratio at the louder signals than at the weaker or softer signals. Also, when the frequency swing is small, representative of, for example, soft music, the entire pass band of the receiver is not usefully employed and the pass band of the receiver in excess of that necessary to instantaneously pass the signaling frequencies may permit passage of impulse noises or atmospheric static during soft or weak signal reception. In this way in ordinary frequency modulation systems, where adjusted for narrower or wider swings, the signal to noise ratio is at its worst during soft music when it should be at its best. To improve this situation and provide a receiving system which has a better signal to noise ratio during softer or weaker signals is an object of my present invention. This object is effected by providing means for automatically adjusting the pass band of the receiver in accordance with the frequency swing or loudness of the signals. Thus, when the frequency swing is small and the signal transmitted may be represented by soft music, the pass band of the receiver is made smaller.

Asshown in Figure l the antenna 300 feeds a rst tuned radio frequency amplifier stage 302'. For simplicity, polarizing and biasing sources have been omitted from Figure 1. The output circuit of stage 392 is broadened by resistance 30e and feeds into a limiter 306 whose output level is of constant value. lThe output of limiter 396 is fed into the analyzing circuit 308 which has the characteristic shown in Figure 4. That is, analyzer 308 may be a single off-tuned cir-'- cuit converting frequency modulating currents into currents of varying amplitude. The rectangle 388 is designated as Discrminator to indicate the latter function. These currents of varying amplitude are fed into a detector 310' whose audio output in turn is fed to an audio frequency amplifier 3l2. Connected to the audio frequency amplier are the earphones or loudspeaker 3l4'. Tube SI5' acts as a variable resistance shunting the input circuit 3i8 of the first radio frequency stage 302. Part of the output of the discrirninator circuit is fed to a rectifier 32D provided with asuitable time constant circuit 322 which may be adjusted to be fast or slow as found satisfactory. The output of rectier 320 thereby varies the conductivity of tube 316' making it more conductive in the case of louder signals and thereby widening the pass band of the first stage 302'. When soft signals come through in which there is only a small frequency swing, tube 3I6 becomes less conductive and acts as a higher resistance in shunt to circuit 3l8. Hence, the pass band of the receiver during weaker signals or softer passages becomes sharper, thereby reducing noise passed by the rst stage of the receiver.

This action is illustrated in Figure 6 where the curve marked N illustrates the narrow frequency band passed by the tuned circuit 618 of Figure 1 when soft music is being received and when tube 3|6 is cut offor .practically cut olf. With louder signals, tube .3l6' becomes conductive and hence broadens thepassband of circuit SiS as shown by curve W of Figure.' 6. The changes in amplitude produced by changing the pass band of circuit `318 have no .effect onthe ultimate receiver output in loudspeaker V3|4' because of the fact that limiter 386' serves to eliminate these undesirable amplitude changes.

VIn Figure 5,'I have illustrated a receiver employing the features explained in connection with Figures 1 and 6. In addition, supplementary control is provided bychange in coupling as shown in Figure 7. Thus, when the coupling between stages or circuits is loosened, the pass band may vary from one extreme, as illustrated by curve WC of Figure 7, to a pass band illustrated by curve NC of Figure 7.

Turning to Figure 5, the antenna 468 feeds through `coupling coil 452 `shunted by resistance 484 the ,first R.. F. amplifying stage 486. Stage 406 successively feeds amplifying stage 468, the latter in turn 'feeding a converter 418 also fed with`local oscillations from a local source 4|2. The resultant beat frequency energy is fed through beat frequency amplifier stages 414, 416 to a limiter 4l8. The network 420 is a discriminator, and may be constructed as described in Seeley Patent 2,121,103. The variable amplitude currents derived from circuits 428 are fed to the push-pull detector tubes 422, the latter in turn feeding an audio frequency amplier 424 and earphones or loudspeaker 426.

Part of 'the audio frequency energy, or video frequency energy in case television signals have been transmitted, is fed through leads 428 to rectier 430 and thence to time constant circuit 432 which may be slow or fast as found desirable and is preferably made fast. Part of the voltage developed across the resistor of circuit 482 is fed to the D. C. amplifier 434 energizing solenoid 436, the latter pulling the core 438 against the action of spring 440. Hence, as the audio output from rectiers 422 becomes larger, solenoid 43S Sucks the core 438 to the right, thereby moving the way, the pass band of the receiver is increased by not only broadening the circuits with resistance Vby the'action of tubes 468, but also by increasing Athe coupling between stages. If desired, of course,

the increased coupling feature may be removed by opening .switch 480. Alternatively, the receiver system may be set for narrow band operation by moving switch 482 from its position shown so as to contact with point 484 of blocking voltage source 486, which is of sufiiciently high value to block and prevent all current flow through the resistance tubes 468.

All of the resistance tubes 468 are identical. Hence, only the complete connections for the first tube tothe far left are illustrated. The connections 4for the remaining tubes are identical and hence need not be illustrated or further explained. Tubes 468 may be provided with screen grids 580, a source of plate Vpotential 582, voltage dropping resistors 504, choke coils 586 and bypassing condensers 508.

All of the frequency modulationreceivers described herein may `be used to receive phase modulated waves, it being understood that, preferably, an additional correction circuit is added to the signal or audio frequency amplifier stages ofthe receiver. This .additional correction should have a characteristic such that the amplifier output falls off with increase in frequency. The preferred correction circuit for the receivers describedV herein when used to receive phase modu lated waves is:described in Crosby Patent No. 2,060,611.

.Inconnection with the described receivers, as explained in connection with Figures 2 and 3, it is preferred that the narrowest adjustment or narrowest passY band of the receiver be equal to and no less than twice the highest audio or modulating frequency. This follows from the fact that inV frequency modulation the band Width necessary vis the higher of two factors, namely, the highest modulating frequency or the highest swing in frequency of the radiated or carrier frequency.

'In `thereceiving systems of Figures 8 and 9, the reeciving apparatus within rectangles 8 and 9 andlbetween antenna 400 and loudspeaker or earphones 426 is identical to that shown in Figure 5 with the exception that the apparatus within the dotted rectangle I8 of Figure 5 is omitted and the controlling voltages are applied to leads 602, 600. in different ways.

In Figure 8 part of the waves picked up on antenna 400 are fed through the broadly tuned coupling system I2 to a radio frequency amplifier I4. The output of Vamplier I4 is beat down in the first detect0r.|6 -Withwaves from the local oscillator I8. The beat frequency waves are fed through an intermediate frequency amplifier and limiter 20. TheV output of unit 28 is fed into a second `unit 22 which converts the frequency modulated waves into waves of varying amplitude. This unit 22 may be of the type shown in rectangle 4,20 Of ,Figure-5 or it may be the off- Overlapping resonance CUIVGS.

tuned circuit system showr'r'inV either' Conrad are tapped as shown. As illustrated, the couplingV circuit I2 is unicontrolled with the tuning of the receiving system 8. Resistors 30, 32 are provided, as illustrated, so that for all tunings of the receiver radio frequency amplifier I4 is enabled to receive the largest changes in frequency transmitted. Similarly, the circuits within units I E, 20, 22 and 24 are broadened with resistance so as to accommodate the largestV frequency changes received.

The system of Figure 9 is a simplified arrangement of the receiver of Figure 8. In the system of Figure 9 unit 90 is a radiofrequency amplifier and limiter whose output is fed into a discriminator circuit 22 already described in connection with Figure 8. Unit 22 of Figure 9 is followed by a rectifier or detector 42 whose output is fed into the time constant circuit 28 supplying voltages to the controlling leads 600, 602. If desired, of course, another rectifier following 42 and ahead of circuit 28 may be provided. K

All of the remarks made in connection with Figure 5 are, of course, equally applicable to Figures 8 and 9. For example, switch 480 of Figures 8 and 9 may be opened or switch 480 may be closed and switches 482 moved over to contacts 484.

In the receiver of Figure 10, frequency modulated waves are picked up by antenna 2c and fed through tunable broad pass coupling 4c to radio frequency amplifier 6c. The output of radio frequency amplifier 5c is fed to the first detector 8c which is also supplied with oscillations from local oscillator |00. Beat frequency energy is fed into the intermediate frequency 'amplifier and limiter stages |60. Radio frequency amplifier 6c is provided with a detector or rectifier I2C, operating as an automaticV volume controlling rectifier whose output, through a suitable time constant circuit Mc, controls the gain of radio frequency amplifier 6c. If desired, the input leads 20c may be connected to the output leads of the first detector 8c. In either event, the automatic volume control leads 24e may be connected as shown or they may be connected to amplifier 29e to vary the energy fed from oscillator l0c to the rst detector. If desired, leads c may be connected to both radio frequency amplifier 6c and to amplier o to simultaneously control the gain or output thereof. The purpose of having the energy fed to the limiters of the I. F. stages volume controlled is to prevent the limiters from being subjected to wide variations whereby the output of the I. F. limiter stages to the input side of detectors 30C and 34o is substantially constant and devoid of drooping levels following from under or over excitation of limiters.

Circuits 32e and 35C are tuned to opposite sides of the mean intermediate frequency and have The outputs of detectors 30e and 34C are connected in phase opposition and fed through transformer 38C to audio frequency amplifier 40e connected to a loudspeaker 42o.

For high fidelity reception, audio frequency amplifier 40c and loudspeaker 42a are made substantially fiat from about 20 to 15,000 cycles.

Since the higher notes above 8,000 or 9,000 cycles to 15,000 cycles are present only in infrequent high character programs such as presented by philharmonic orchestras, etc., I provide an arrangement to keep high frequency noise lbackground out of the loudspeaker when ordinary'run of the mill programs are received.

To do this I connect a high pass filter which passes solely 8,000 to 15,000 cycles across the input side of the audio amplifier 40e. The Youtput of filter 44e is fed to rectifier 40e feeding the timeY constant circuit 48e. Bias voltage source 09e is chosen so that in the presence of only high frequency back ground noise which is of low level, tube 50c is substantially conductive. Filter 54o is arranged to freely pass energy from 8,000 to 15,000 cycles and, hence, effectively grounds the loudspeaker 42e for these noise frequencies through Icy-passing condenser 52e and normally conducting tube 50c. musical frequencies strong enough to override the back ground noise, filter 44o passes enough of such energy to rectifier 46c to bias the grid of tube 50c negatively to a higher degree and thus .cause tube 50c to4 become non-conductive.

Hence, filter 54C is effectively open circuited and the higher frequencies are no longer shunted away from, but are permitted to flow into, loudspeaker 42e.

If desired a switch 55o may be provided to render inoperative this feature of Figure 10.

When receiving on shorter wave lengths, the swing ofthe carrier may be quite large, numerically. Thus, for transmitting an audio signal on a frequency modulated carrier of mean wave length of 65 centimeters, the loudest sound transmitted may cause a swing of the carrier of plus and minus 400 k. c. The frequency characteristic 'or slope of circuits 32e and 35e will not be great and, hence, the voltage drop across @0c due to small changes in frequency caused by frequency drift will be small and generally7 ineffectual to frequency control the local oscillator I0c.

This situation may be remedied in either of two Y. ways. First, a part of the intermediate frequency energy is fed through transformer 20c and then through filter 84o which has a pass band greater than the permissible drift range of oscillator I0c. The energy passed by filter 84e is fed to frequency multiplier 85o which has a multiplying factor such as desired, for example, the factor may be any number from 2 to 50. The output of multiplier 85o is fed to relatively sharply tuned circuits 30a and 82e having overlapping resonance curves and tuned symmetrically to opposite sides of a mean frequency corresponding to the desired correct mean frequency of I. F. amplifier Ito multiplied by the multiplying factor of multiplier 86e. As a result small changes in frequency cause relatively large correcting voltages across resistor 92e, which through switch 84e and leads Sie are fed to the frequency controlling circuit 98C for oscillator I 0c. In this way, oscillator I0c is quickly brought to its correct operating frequency or to a frequency producing a correct intermediate frequency should the transmitter mean frequency vary.

In the alternative, a direct current amplifier 16o may be provided which amplifies small changes in voltage appearing across the taps for leads 6Ic. By throwing switch 94e to contacts 95o, the amplified output of 16e is fed to leads 91e and thence to the frequency controlling circuit 98c for oscillator I0c.

In either frequency controlling case, it may However, in the presence of be found desirable to automatically.suppressfrequency control when the'signal is on and to have it operate only during silent passages in a received program. This is accomplished by rectifying part of the signal fed Vthrough leads lc to rectifier 62e. The rectified output of rectifier 62e impressed through resistor c and through leads 18e and 14e on the grids of tubes 88o, 90C and rectifier o block the amplifier and tubes. In the absence of audio signal or when the audio signal drops to a low value, automatic frequency control of oscillator l0c again takes place. If it is desired to remove the suppressing action referred to, switch 68e may be opened.

It is to be clearly understood that all of the features, such as the audio stage noise suppression and automatic frequency control features, disclosed in Figure 10 may be combined with and included in the receiving systems described in Figures 5, 8 and 9. Because such combined wiring diagrams cannot be placed upon a single sheet of drawings, I have not illustrated the combined system in a single figure. Moreover, it should be understood in connection with Figure 10 that the pass band 8,000 to 15,000 cycles is merely given by way of example. Other limiting values may be chosen. For example, the overall pass band for the audio frequency stages may be from l0 to 10,000 cycles. High frequency noise elimination in the absence of signal over-tones may be made to take place in the band from '1,000 to 10,000 cycles, for example.

Having thus described my invention, what I claim is:

1. In a frequency modulated carrier wave receiver of the type comprising a plurality of cascaded carrier wave amplifiers, a carrier-tuned selector network coupling each pair of cascaded amplifiers, and at least one of the selector networks consisting of a pair of coupled tuned circuits resonated to the center frequency of a modulated carrier wave, means for deriving modulation voltage from the amplied received carrier wave, the improvement which includes a device connected with at least one of said tuned circuits for 1.controlling the damping thereof,a second device for controlling the degree of coupling between said pair of tuned circuits, and further means for :deriving from said modulation voltage a unidirectional voltage representative of the extent of carrier Vfrequency deviation of the received modulated carrier waves, said further deriving means controlling said devices in a sense directly to synchronize the pass band width of said pair of tuned circuits with said carrier frequency deviation.

2. In a frequency modulation receiver of the superheterodyne type'including a signal collector, a converter, an intermediate frequency amplier including at least one pair of coupled resonant circuits tuned to the intermediate frequency, an amplitude modulation limiter, means for deriving modulation voltage from the amplified frequency-modulated, intermediate frequency voltage; the improvement which comprises means operatively associated with said intermediate frequency amplifier tuned circuits for adjusting the coupling therebetween and thereby the effective pass'band width of said amplifier, means for deriving from the said modulation voltage a unidirectional control voltage whose magnitude is directly proportional to the modulation voltage amplitude, and means utilizing said control voltage for regulating said coupling adjusting means.

HARRY TUNICK.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,819,508 Hansell Aug. 18, 1931 2,010,131 Beers Aug. 6, 1935 2,040,955 Roberts May 19, 1936 2,140,391 Mayer Dec. 13, 1938 2,170,202 Kupfmuller Aug. 22, 1939 2,184,072 Freeman Dec. 19, 1939 2,205,762 Hansell June 25, 1940 

